Toroidal continuously variable transmissions that are used as automatic transmissions for automobiles are disclosed in many publications such as JP 2001-317601 (A) and “Toroidal CVT”, Hirohisa Tanaka, Corona Publishing Co., Ltd. (Jul. 13, 2000), and such transmissions are well known and are partially being put into use. This kind of toroidal continuously variable transmission includes an input-side disk and an output-side disk that are such that the side surfaces in the axial direction thereof that face each other are toroid shaped curved surfaces, and a plurality of power rollers that is held between these disks. During operation, the rotation of the input-side disk is transmitted to the output-side disk by way of the power rollers. These power rollers are supported by support members such as trunnions so as to be able to rotate freely, and these support members are supported so as to be able to freely pivot and displace around pivot shafts that are located offset from the center axis of the input-side and output-side disks such that the pivot shafts and the center axis do not lie in the same plane. When changing the transmission gear ratio between the input-side and output-side disks, a hydraulic actuator causes the support members to displace in the axial direction of the pivot shafts. Supply and discharge of hydraulic oil to and from this actuator is controlled by a control valve, however, at the same time, the movement of the support members is fed back to the control valve.
When the support members are caused to displace in the axial direction of the pivot shafts based on the supply and discharge of hydraulic oil to and from the actuator, the direction of the force in the tangential direction that acts at the areas of contact (traction sections) between the circumferential surfaces of the power rollers and the side surfaces of the input-side and output-side disks changes, and side slipping occurs in the areas of rolling contact. As the direction of this force changes, each of the support members pivots (inclines) around the respective pivot shaft, and the locations of contact between the circumferential surfaces of the power rollers and the side surfaces of the input-side and output-side disks change. When the circumferential surfaces of these power rollers come in rolling contact with the portion of the input-side disk that is near the outside in the radial direction and the portion of the output-side disk that is near the inside in the radial direction, the transmission gear ratio between the input-side and output-side disks is on the accelerating side. On the other hand, when the circumferential surfaces of these power rollers come in rolling contact with the portion of the input-side disk that is near the inside in the radial direction and the portion of the output-side disk that is near the outside in the radial direction, the transmission gear ratio between the input-side and output-side disks is on the decelerating side.
When this kind of toroidal continuously variable transmission device is assembled in an automatic transmission of an automobile, construction in which the continuously variable transmission is combined with a differential gear unit such as a planetary gear mechanism has been proposed. JP 2003-307266 (A) discloses a continuously variable transmission device in which the input shaft is rotated in one direction, and the rotating state of the output shaft is switched between a forward rotating state and a reverse rotating state with the stopped state (so-called gear-neutral state) in between. In the case of this kind of continuously variable transmission device, in the so-called low-speed mode state, the transmission gear ratio of the overall continuously variable transmission device changes to become infinitely large. In other words, by adjusting the transmission gear ratio of the toroidal continuously variable transmission, while the input shaft remains in a state of rotating in a single direction, the rotating state of the output shaft can be changed between a forward rotating state and reverse rotating state with a stopped state in between. In the case of a continuously variable transmission device that is capable of achieving an infinitely large transmission gear ratio, the transmission gear ratio of a toroidal continuously variable transmission is such that near a value where it is possible to achieve a stopped state of the output shaft (geared neutral point, GN point), the state of the power that is transmitted to the output shaft greatly changes even when this transmission gear ratio is changed only a little. Therefore, control of the transmission gear ratio of a toroidal continuously variable transmission must be performed with high precision.
For example, when the automobile is in the stopped state and the shift lever is moved from a non-moving state such as the P range (parking position) or N range (neutral position) to a moving state such as the D range (normal forward position), L range (high drive forward position) or R range (reverse position), a suitable driving force in the forward or reverse direction is quickly generated and it is necessary to keep the vehicle in the stopped state by a braking force caused by operating the brake pedal. Therefore, in a state in which the shift lever has selected a non-moving state, the transmission gear ratio of a toroidal continuously variable transmission must be strictly controlled at a value at which it is possible to achieve a state of an infinitely large transmission gear ratio. Supposing that the transmission gear ratio of a toroidal continuously variable transmission shifts a large amount from a value at which it is possible to achieve an infinitely large transmission gear ratio, and the shift lever has selected a moving state, there is a possibility that a driving force that is greater than anticipated (creep force) will be transmitted and the vehicle will begin to move, or that a driving force in a direction opposite that intended by the operator will be transmitted.
On the other hand, there is a large number of parts assembled in a toroidal continuously variable transmission, and the dimensional precision and assembly precision of many of those parts have an effect on the transmission gear ratio of the toroidal continuously variable transmission. Therefore, it is feasible that individual differences will occur in the transmission gear ratio of toroidal continuously variable transmissions that are capable of achieving a state of an infinitely large transmission gear ratio that is found through design calculation. Moreover, it is also feasible that the characteristics of a transmission gear ratio of a toroidal continuously variable transmission that is capable of a state of an infinitely large transmission gear ratio will change due to changes over time of the components that are used for long periods of time (slight plastic deformation).
On the other hand, JP 2004-308853 (A) discloses giving a learning function to a controller for learning the step position of a stepping motor, in which with the shift lever selecting a non-moving state as a condition, the output shaft is stopped with the input shaft rotating as is. More specifically, with the shift lever selecting a non-moving state as a condition, the rpm of the input-side disk and the rpm of the output-side disk of a toroidal continuously variable transmission are detected by respective rotation sensors. The controller finds the rotational speed of the output shaft in a non-moving state based on the actual transmission gear ratio that is obtained from the rotational speeds of the input-side disk and output-side disk (rotational speed of the input-side disk/rotational speed of the output side disk), and the transmission gear ratio of a planetary gear transmission. The controller then adjusts the transmission gear ratio of the toroidal continuously variable transmission by adjusting the step position (driving amount) of the stepping motor so that the rotational speed of the output shaft is “0”. The controller learns the step position for the state where the rotational speed of the output shaft is “0”, then stores that step position in the controller memory to complete learning control. The controller controls the transmission gear ratio of the toroidal continuously variable transmission with the adjusted step position (learned value) as a reference. As a result, it becomes possible to control the transmission gear ratio with high precision without being affected by individual differences in or changes over time of the components of the toroidal continuously variable transmission.
However, in the case of the control method related to learning the step position that was conventionally considered, there is a possibility that when the driver turns OFF the ignition switch (ignition key) during learning of the step position, the controller will no longer be able to accurately learn the step position. In other words, when the user turns OFF the ignition switch in order to stop the engine from running (operating), the engine rotational speed drops over a certain period of time, although it is a short period of time. The speed of this drop is fast, so in this state, there is a possibility that the drop in the rotational speed of the input-side and output-side disks will not be synchronized with each other. Therefore, there is a possibility that the transmission gear ratio of a toroidal continuously variable transmission, which is calculated from the rotational speeds of the input-side and output-side disks, will be off from the value of the actual transmission gear ratio. When learning of the step position continues in a state such as this in which it is not possible to accurately calculate the transmission gear ratio of the toroidal continuously variable transmission, a problem occurs in that the obtained learned value is off from a suitable position for stopping the output shaft, and the step position will be incorrectly learned.
Moreover, when the rotation of the engine (crankshaft) is transmitted as is to the input-side disk, it is feasible, as a way to reduce costs, to omit the input-side rotation sensor for detecting the rotational speed of the input-side disk, and calculate the transmission gear ratio of the toroidal continuously variable transmission using a signal from an engine controller that expresses the engine rotational speed. However, in this case, it is also possible that at the same time that the driver turns OFF the ignition switch, the signal that expresses the engine rotational speed will no longer be obtainable, so it will become difficult to accurately calculate the transmission gear ratio of the toroidal continuously variable transmission. Therefore, there is a possibility that the obtained learned value will be off from a suitable position for stopping the output shaft.
When the ignition switch is turned OFF in this way during learning of the step position, there is a possibility that the transmission gear ratio of the toroidal continuously variable transmission will not be able to be calculated accurately regardless of the calculation method used for calculating the transmission gear ratio of the toroidal continuously variable transmission. As a result, there is a possibility that transmission gear ratio control will start in a state in which the step position of the stepping motor is off from the accurate position for stopping the output shaft, so not only is there a possibility that the feeling when shifting will be impaired, but in the worst case, there is also a possibility that the vehicle will move in a direction opposite the position selected by the shift lever.
On the other hand, learning of the step position can only be executed when the engine is running, so, it is possible to simply add the condition that the engine rotational speed is not “0” (zero), or the condition that the engine rotational speed is higher than a specified rotational speed to conditions for allowing learning of the step position. However, in this case as well, even though there is a possibility that the occurrence frequency of the problem described above can be reduced, learning of the step position is executed while the engine rotational speed goes from the idling speed (for example 800 rpm) to “0”, or while the engine rotational speed drops from the idling speed to a specified rotational speed or less (for example, when the specified rotational speed is set to 500 rpm, range D in FIG. 4), so basically the problem described above cannot be solved.
Furthermore, by setting the specified rotational speed to a value close to about 90% of the idling speed, it is feasible that the time that the conditions for allowing learning of the step position may be satisfied while the engine rotational speed drops after the ignition switch is turned OFF will be reduced, and that the learning time leading to erroneous learning will be shortened. In this case, it is possible to reduce the occurrence frequency of the problem described above, however, the idling speed fluctuates depending on the temperature of the engine coolant and the like, so there is a possibility that the conditions for allowing learning will not be satisfied even though the ignition switch is ON and the engine rotational speed has not dropped and is within the original range of being an object of learning control, and thus there is a possibility that a new problem will occur in which opportunities for learning are lost more than necessary, and that the frequency that learning control is performed will be reduced more than necessary.